The present invention relates to solid polymer fuel cells that are rendered more tolerant to voltage reversal through modifications to the anode structure near or in the catalyst layer.
Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of economically delivering power with environmental and other benefits. To be commercially viable however, fuel cell systems need to exhibit adequate reliability in operation, even when the fuel cells are subjected to conditions outside the preferred operating range.
Fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include solid polymer electrolyte fuel cells that comprise a solid polymer electrolyte and operate at relatively low temperatures.
A broad range of reactants can be used in solid polymer electrolyte fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
During normal operation of a solid polymer electrolyte fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
Solid polymer electrolyte fuel cells employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d), which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Separator plates, or flow field plates for directing the reactants across one surface of each electrode substrate, are disposed on each side of the MEA.
Each electrode contains a catalyst layer, comprising an appropriate catalyst, which is located next to the solid polymer electrolyte. The catalyst may be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The catalyst layer typically contains ionomer, which may be similar to that used for the solid polymer electrolyte (for example, up to 30% by weight Nafion(trademark) brand perfluorosulfonic-based ionomer). The catalyst layer may also contain a binder, such as polytetrafluoroethylene.
The electrodes may also contain a substrate (typically a porous electrically conductive sheet material) that may be employed for purposes of reactant distribution and/or mechanical support. Optionally, the electrodes may also contain a sublayer (typically containing an electrically conductive particulate material, for example, finely comminuted carbon particles, also known as carbon black) between the catalyst layer and the substrate. A sublayer may be used to modify certain properties of the electrode (for example, interface resistance between the catalyst layer and the substrate).
Electrodes for a MEA can be prepared by first applying a sublayer, if desired, to a suitable substrate, and then applying the catalyst layer onto the sublayer. These layers can be applied in the form of slurries or inks, which contain particulates and dissolved solids mixed in a suitable liquid carrier. The liquid carrier is then evaporated off to leave a layer of particulates and dispersed solids. Cathode and anode electrodes may then be bonded to opposite sides of the membrane electrolyte via application of heat and/or pressure, or by other methods. Alternatively, catalyst layers may first be applied to the membrane electrolyte with optional sublayers and substrates incorporated thereafter, either on the catalyzed membrane or an electrode substrate.
In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, numerous cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. (End plate assemblies are placed at each end of the stack to hold it together and to compress the stack components together. Compressive force is needed for effecting seals and making adequate electrical contact between various stack components.) Fuel cell stacks can then be further connected in series and/or parallel combinations to form larger arrays for delivering higher voltages and/or currents.
However, fuel cells in series are potentially subject to voltage reversal, a situation in which a cell is forced to the opposite polarity by the other cells in the series. This can occur when a cell is unable to produce the current forced through it by the rest of the cells. Groups of cells within a stack can also undergo voltage reversal and even entire stacks can be driven into voltage reversal by other stacks in an array. Aside from the loss of power associated with one or more cells going into voltage reversal, this situation poses reliability concerns. Undesirable electrochemical reactions may occur, which may detrimentally affect fuel cell components. Component degradation reduces the reliability and performance of the fuel cell.
The adverse effects of voltage reversal can be prevented, for instance, by employing diodes capable of carrying the stack current across each individual fuel cell or by monitoring the voltage of each individual fuel cell and shutting down an affected stack if a low cell voltage is detected. However, given that stacks typically employ numerous fuel cells, such approaches can be quite complex and expensive to implement.
Alternatively, other conditions associated with voltage reversal may be monitored instead, and appropriate corrective action can be taken if reversal conditions are detected. For instance, a specially constructed sensor cell may be employed that is more sensitive than other fuel cells in the stack to certain conditions leading to voltage reversal (for example, fuel starvation of the stack). Thus, instead of monitoring every cell in a stack, only the sensor cell need be monitored and used to prevent widespread cell voltage reversal under such conditions. However, other conditions leading to voltage reversal may exist that a sensor cell cannot detect (for example, a defective individual cell in the stack). Another approach is to employ exhaust gas monitors that detect voltage reversal by detecting the presence of or abnormal amounts of species in an exhaust gas of a fuel cell stack that originate from reactions that occur during reversal. While exhaust gas monitors can detect a reversal condition occurring within any cell in a stack and they may suggest the cause of reversal, such monitors do not identify specific problem cells and they do not generally provide any warning of an impending voltage reversal.
Instead of or in combination with the preceding, a passive approach may be preferred such that, in the event that reversal does occur, the fuel cells are either more tolerant to the reversal or are controlled in such a way that degradation of any critical hardware is reduced. A passive approach may be particularly preferred if the conditions leading to reversal are temporary. If the cells can be made more tolerant to voltage reversal, it may not be necessary to detect for reversal and/or shut down the fuel cell system during a temporary reversal period.
During voltage reversal, electrochemical reactions may occur that result in the degradation of certain components in the affected fuel cell. Depending on the reason for the voltage reversal, there can be a rise in the absolute potential of a fuel cell anode. This can occur, for instance, when the reason is an inadequate supply of fuel (that is, fuel starvation). During such a reversal in a solid polymer fuel cell, water present at the anode may be electrolyzed. When significant water electrolysis can occur, the fuel cell voltage typically remains above about xe2x88x921 V, but this voltage depends on several variables including the amount of water present, the amount of fuel present, current drawn, and temperature. It is preferred to have electrolysis occur rather than component oxidation. When water electrolysis reactions at the anode cannot keep up with the current forced through the cell, the absolute potential of the anode can rise to a point where oxidation (corrosion) of anode components takes place, thereby typically irreversibly degrading the components. Therefore, a solid polymer fuel cell can be made more tolerant to voltage reversal by increasing the amount of water available for electrolysis during reversal, thereby using the current forced through the cell in the more innocuous electrolysis of water rather than the detrimental oxidation of anode components. By increasing the amount of water in the vicinity of the anode catalyst during normal operation, more water is available at the anode catalyst in the event of a reversal. Thus, modifications to the anode structure that result in more water being present at the anode catalyst during normal operation lead to improved tolerance to voltage reversal.
In a typical solid polymer fuel cell, water generated at the cathode diffuses through the polymer membrane to the anode. By restricting the passage of this water through the anode structure and into the exhaust fuel stream, more water remains in the vicinity of the catalyst. This can be accomplished, for example, by making the anode catalyst layer or an anode sublayer impede the flow of water (either in the vapor or the liquid phase). For instance, adding a hydrophobic material such as polytetrafluoroethylene (PTFE) to either of these layers makes them more hydrophobic, thereby hindering the flow of water through the anode. Alternatively, other additives (for example, graphite, other carbon, or titanium oxide powders) may be employed that serve to reduce the porosity of either layer thereby impeding the flow of water through the anode. In certain embodiments, it may be advantageous to employ sufficient porosity reducing additive to occupy from about 0.1 to 0.2 xcexcL volume per cm2 of the anode catalyst layer. In particular, it may be advantageous to employ a porosity reducing additive comprising a mixture of polytetrafluoroethylene and acetylene carbon black in which the anode catalyst layer comprises between about 12% and 32% by weight of polytetrafluoro-ethylene and between about 0.03 and 0.2 mg/cm2 of acetylene carbon black.
Another approach to increase the amount of water in the anode catalyst layer is to increase the water content in the catalyst layer components. A conventional catalyst layer for instance may contain up to 30% by weight amount of fully hydrated perfluorosulfonic ionomer with 1100 equivalent weight. Thus, an increase in water content can be accomplished by increasing the amount of the water containing ionomer used in the catalyst layer or by employing a different ionomer with higher water content (for example, trifluorostyrene, instead of perfluorosulfonic-based ionomers such as Nafion(trademark)). Alternatively, more hygroscopic materials, such as Shawinigan acetylene black carbon, may be incorporated in the anode catalyst layer to retain more water therein.
Suitable modifications to the anode structure therefore include the use of a different component or the use of a greater amount of a component, in either the catalyst layer or a sublayer, than is conventionally employed. The modification may result in a performance trade-off in another fuel cell performance characteristic, such as power density.